The invention concerns a fuel cell with an anode and a cathode and a polymer electrolyte arranged between them, said anode being in contact with a fuel cell distribution system having an inlet and an outlet so that fuel can penetrate to the anode from said system, said fuel distribution system being arranged in an anode circulation system containing a methanol/water mixture and with a first metering device for supplying methanol to the anode cycle.
Such fuel cells are called direct methanol fuel cells (DMFC) because methanol is supplied directly to the anode and is oxidized there. This procedure has the advantage over a fuel cell operated with hydrogen at the anode that a separate device for reforming the methanol into hydrogen can be omitted. The problem of this technology, however, is the fact that until now no membrane totally impermeable to methanol has been available.
In the previously used membranes permeable for methanol, a methanol loss flow appears, the so-called methanol crossover, which reduces the power and efficiency of the DMFC. In order to keep the crossover as small as possible, the methanol concentration in the methanol/water mixture is adjusted to a relatively low level, on the one hand, and on the other hand, it is adapted to the electric power consumption. However, several problems arise in this case especially in the case of rapid load changes, which are important above all when the DMFC is used to drive a motor vehicle.
These problems also arise whenever hydrocarbons other than methanol are used. For this reason in this application, methanol is used as a representative for these other hydrocarbons so that the scope of protection should not be considered to be limited to the use of methanol.
The above mentioned adaptation of the concentration to the power taken from the fuel cell is essentially proportional: for a high electric power consumption the concentration is raised and for a lower power consumption the concentration is lowered again. The concentration can be raised relatively simply by adding pure methanol to the anode cycle (a.k.a. fuel supply circuit) which enables a rapid adaptation in time. The concentration cannot be lowered so quickly. It is therefore achieved by stopping the methanol supply in such a case so that the methanol is gradually consumed in the anode cycle. In this case the concentration changes only slowly, however, for the following reasons: for a homogeneous methanol concentration inside the fuel cell, it is necessary for the methanol to be supplied to the anode in a highly superstoichiometric ratio. This means that when the methanol/water mixture passes through, only a small portion of the methanol is oxidized at the anode. This in turn has the result that the methanol concentration is not changed substantially between the inlet and the outlet of the gas distribution structure, which means that for several cycles the methanol/water mixture concentration is not adapted to the power consumed.
A methanol fuel cell of a fuel cell system according to the general concept of claim 1 is described in the article by Manfred Waidhas: xe2x80x9cMethanol-Brennstoffzellenxe2x80x9d (Methanol Fuel Cells) published in Brennstoffzellen: Entwicklung, Technologie, Anwendung/Konstantin Ledjeff (editor), first edition, Heidelberg: Mxc3xcllelr, 1995 ISBN 3-7880-7514-7, pages 137, 148. The fuel cell system described there, to be sure, is used for experimental purposes. It is therefore not designed for rapid load changes since no rapid changes in the methanol concentration are necessary in experimental operation. The methanol is supplied by a metering pump (FIG. 10-9) to the anode cycle.
In order to solve the above presented problem, i.e. to design the fuel cell system in such a way that the concentration of methanol in the anode cycle can be rapidly adapted to a varying electric power consumption, it is proposed that a device (hereinafter methanol-separator) be present in the anode cycle of a fuel cell with the features of the generalizing part of claim 1 for reducing the content of methanol and that the first metering device be connected to the inlet segment of the anode cycle between said methanol-separator and the inlet to the fuel distribution system at the anode. This arrangement makes possible the following process for controlling the system:
The methanol-separator (i.e. device for reducing the methanol content) holds the concentration at a relatively low level at all times, and the concentration of methanol in a mixture with a relatively low concentration can be increased by supplying methanol thereto before the inlet to the fuel cell. If a high concentration is necessary in the fuel cell system, a large amount of methanol is supplied; in the case when a low concentration is required, if necessary, the mixture of methanol can be totally stopped.
The anode and cathode of the fuel cell are connected to each other by an electric circuit from which variable power can be taken off, the concentration of a first effluent at one outlet of the methanol-separator being set at a value which is suitable for operating a fuel cell in the case of a low power takeoff.
The methanol necessary for supply the anode cycle can, on the one hand, be taken from a methanol tank of the first metering device containing pure methanol, and on the other hand, from a secondary cycle which receives a second effluent comprising a highly concentrated methanol/water mixture accumulating in the methanol-separator in which case a mixture tank present in the secondary cycle is connected via a second metering device to the inlet segment of the anode cycle.
In steady state operation the flow of materials of the anode cycle and the secondary cycle are totally mixed with each other again. In this way at the inlet to the fuel cell, one obtains once more the concentration which was present at the outlet from the fuel cell. The spent methanol is replaced form the methanol tank. If more power is required, the total concentration can be increased by the further addition of methanol from the methanol tank; if less power is needed the metering of methanol from the methanol tank stops. In addition, the secondary cycle is blocked so that only the lean mixture passes from the anode cycle to the fuel cell. If more power is required than corresponds to the lean mixture, at first a richer mixture is fed in from the secondary cycle again. If this is no longer sufficient, pure methanol is again metered in from the methanol tank.
In order to be able to lower the concentration even further if necessary or in order to have means for better controlling the increase in concentration in the anode cycle, the inlet segment is connected via a third metering device to a water tank.
The fuel cell system is now operated in such a way that, depending on the power consumption, the first and second metering devices are controlled in such a way that by supplying methanol through the first metering device or a highly concentrated methanol/water mixture via a second metering device the methanol concentration at the inlet to the fuel cell distribution system can be raised to a value which corresponds to the power demand in each case. This has the result that in steady operation, the material streams of the anode cycle and the secondary cycle are totally mixed with each other once more. In this way, the concentration at the fuel cell inlet is adjusted back to a value which was present at the outlet from the fuel cell. To the extent that methanol is oxidized on the anode, a corresponding amount of methanol is supplied by the first metering device. If the power demand increases, by adding methanol via the first metering device the total concentration can be instantaneously increased.
When the power demand diminishes, the metering through the second metering device is gradually reduced or entirely shut off so that the lean mixture is supplied to the fuel cell at the one outlet of the methanol-separator. If no more power is required at all the concentration can be further reduced instantaneously by supplying water to the anode cycle via the third metering device.
As the device for reducing the methanol concentration, several possibilities exist which may involve a single step or multistep distillation or membrane separating procedures. In these procedures, heat is required which is available in the system itself as waste heat from the fuel cell stack. In this way, additionally, the heat balance of the fuel cell system can be suitable regulated.